Carbon flows in natural plankton communities in the Anthropocene

Abstract

Human activities have large impacts on organisms, ecosystems and biogeochemical cycling. A major human impact is the alteration of the global carbon cycle that lead to a rapid increase in atmospheric CO2 concentrations, which has been partially absorbed by the oceans, altering seawater chemistry and causing ocean acidification. Another pertinent environmental problem is eutrophication of aquatic systems, causing degradation of water quality and ecosystems, especially in lakes. These environmental problems affect the base of the food web, formed by plankton communities. Phytoplankton are the primary producers, converting CO2 and nutrients into organic matter and organic matter is respired by zooplankton (classical food web) and bacteria (microbial loop). The balance between production and community respiration often determines if aquatic systems are sources of sinks for CO2. In this thesis carbon flows in plankton communities were studied, both in marine systems in relation to (ocean) acidification, and in freshwater systems with respect to eutrophication. The methodology was carbon stable isotope (δ13C) analyses of organisms, both in natural abundances and in deliberate tracer additions. δ13C of fatty acids were used to determine the isotopic composition of phytoplankton and bacteria. Ocean acidification effects were studied in mesocosm experiments, described in chapter 2 and 3, where CO2 was added in combination with enriched δ13C as a tracer to quantify the carbon flows within the plankton communities. Plankton community interactions along a trophic gradient in lakes were studied in chapter 4 using natural abundance isotope ratios. Eutrophication often leads to cyanobacterial blooms and their role in the planktonic food web was studied in chapter 5. Macrophyte planting in lakes can be used to combat eutrophication effects and their role in the planktonic food web were studied in chapter 6.